linux-kernel - Re: [PATCH] x86: xen: remove the use of VLAIS

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Date:   Tue, 2 Jan 2018 09:18:40 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Nick Desaulniers <nick.desaulniers@...il.com>
Cc:     ghackmann@...gle.com, mka@...gle.com, kees@...gle.com,
        srhines@...gle.com, Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: xen: remove the use of VLAIS

On 24/12/17 19:02, Nick Desaulniers wrote:
> Variable Length Arrays In Structs (VLAIS) is not supported by Clang, and
> frowned upon by others.
> 
> https://lkml.org/lkml/2013/9/23/500
> 
> Here, the VLAIS was used because the size of the bitmap returned from
> xen_mc_entry() depended on possibly (based on kernel configuration)
> runtime sized data. Rather than declaring args as a VLAIS then calling
> sizeof on *args, we can define the variable length array (mask) to be a
> pointer, and calculate the appropriate sizeof args manually. Further, we
> can get rid of the #ifdef's and rely on num_possible_cpus() (thanks to a
> helpful checkpatch warning from an earlier version of this patch).

Using a pointer for mask seems to be wrong, as it is never initialized.

Why don't you just use:

DECLARE_BITMAP(mask, NR_CPUS);

and drop the #ifdef, while keeping the manual length calculation?


Juergen

> 
> Signed-off-by: Nick Desaulniers <nick.desaulniers@...il.com>
> ---
>  arch/x86/xen/mmu_pv.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
> index 4d62c07..966976c 100644
> --- a/arch/x86/xen/mmu_pv.c
> +++ b/arch/x86/xen/mmu_pv.c
> @@ -1325,20 +1325,18 @@ static void xen_flush_tlb_others(const struct cpumask *cpus,
>  {
>  	struct {
>  		struct mmuext_op op;
> -#ifdef CONFIG_SMP
> -		DECLARE_BITMAP(mask, num_processors);
> -#else
> -		DECLARE_BITMAP(mask, NR_CPUS);
> -#endif
> +		unsigned long *mask;
>  	} *args;
>  	struct multicall_space mcs;
> +	const size_t mc_entry_size = sizeof(args->op) +
> +		sizeof(*args->mask) * BITS_TO_LONGS(num_possible_cpus());
>  
>  	trace_xen_mmu_flush_tlb_others(cpus, info->mm, info->start, info->end);
>  
>  	if (cpumask_empty(cpus))
>  		return;		/* nothing to do */
>  
> -	mcs = xen_mc_entry(sizeof(*args));
> +	mcs = xen_mc_entry(mc_entry_size);
>  	args = mcs.args;
>  	args->op.arg2.vcpumask = to_cpumask(args->mask);
>  
>